1. Field of the Invention
This invention generally relates to a device and method for exposing substrates to a concentrated light source. More particularly, the invention pertains to a light-emitting diode (LED) device for curing substances such as photosensitive sensitive inks, adhesives, and photographic elements. Ultraviolet or visible light is emitted from an array of LED's, and then directed by an array of light guides to a light concentrator having a light input region along its length. The light is then concentrated and emitted from a light output region along the length of the light concentrator to a photosensitive target to be exposed.
2. Description of the Related Art
Electromagnetic energy, particularly energy in an ultraviolet (UV) light frequency range has been found to speed curing of some substances, including fluids such as inks, coatings, and adhesives. Many of these fluids include photoinitiators that convert monomers in the fluids into polymers to solidify the monomer material when the fluids are exposed to UV light. Conventional apparatus for curing substances using UV light sources include lamps and/or LEDs that produce light in a UV frequency range selected to optimize curing times. An LED is a type of electronic semiconductor device that emits light when an electric current passes through it. The light curable photosensitive compositions are then used in the manufacture of electronic components, medical equipment, and other industrial products. In the past, the compositions found in such environments have been cured using high pressure arc lamps to flood the UV sensitive material with UV light. While arc lamp technology is widely used, such technology has several disadvantages. One disadvantage is the relatively short life span of the bulbs used in the conventional arc lamp configured curing sources. Furthermore, the arc bulb degrades nonlinearly during its lifetime. As a result, conventional arc lamp photocuring systems often require monitoring and adjusting the output power as the bulb degrades. Further, arc lamps are typically powered-on even during stand-by periods because they require cumbersome warm-up and cool-down cycles. As a result, much of the life of the conventional bulbs may be lost during these stand-by periods. Another disadvantage is the broad spectrum of the light radiated by the arc lamps. An arc lamp radiates UV, visible, and infrared (IR) light. Typically, UV band pass filters transmit the portion of the UV spectrum required for curing a particular photosensitive material. Heat-rejecting IR filters are usually employed to prevent heating of the cure surface. Because the IR radiation creates a very hot lamp housing, transmission optics near the lamp housing must be made of temperature resistant, UV-transmissive materials. LED's often are substituted for such arc lamps.
U.S. Pat. No. 7,273,369 shows an optical fiber light module which includes a hollow housing including at least one heat sink, and two or more light sources such as LED arrays. U.S. Pat. No. 4,948,214 shows a lens array for optical scanning devices including a light guide and microlens device for LED imaging. U.S. Pat. No. 6,645,230 shows a structure including an array of LEDs mounted on or within a housing. U.S. Pat. No. 7,218,830 shows a flat panel light guide containing several light-guide members and at least one point light source, such as an LED or array of LEDs. U.S. Pat. No. 7,134,768 relates to an LED lamp with light guide, for use in vehicle lamps, traffic signal lamps, video games, and other lighting applications. The structure includes a plurality of LEDs whose light is collimated via a light guide, and inwardly reflected via multiple reflective surfaces within a housing. U.S. Pat. No. 7,194,185 relates to electronic devices having a colored light guide protruding through a cover. The light guide is illuminated by two LEDs below the cover. The two LEDs are in contact with a second light guide, below the cover, which is capable of blending the light colors from the two LEDs. U.S. Pat. No. 6,880,954 shows a method and apparatus for curing photosensitive materials using LEDs and an optical concentrator to generate high optical power intensities. This reference uses both collimated light LEDs and an optical element between the LEDs and an optical fiber. It has been determined that a problem with these aperture arrangements is non-uniformity of light intensity.
The introduction of light emitting diodes has created new alternatives for curing some light sensitive materials. LED technology offers several advantages over the conventional arc lamp technology. Typical LEDs last between 20,000 and 50,000 hours, providing a significant lifespan improvement over arc lamp technology. LEDs also do not emit significant amounts of IR radiation. As an added benefit, the reduced heat generation allows the use of economical light transmitting polymers for optics.
LED sources can also be turned on and off as required because LEDs do not require the warm-up and cool-down periods common in arc lamp systems. Some LED curing systems may implement driver circuits to control the current supplied to the LEDs. These circuits typically use a closed-loop system to monitor and control the output power of the LEDs, by controlling the drive current, to provide a stable and reliable UV/violet source. These circuits may also define different curing cycles for different photosensitive materials, such as emitting a specific output power for a specific length of time.
Unfortunately, conventional LED sources and LED systems have relatively low output power compared to traditional arc lamps. While the lower output power LED photocuring systems have proven to be sufficient for some dental applications, many commercial and industrial light sensitive materials require higher output powers to quickly cure the materials in a fast production environment.
The present invention provides an improved light guide exposure device having improved uniformity and amount of light intensity. The illumination device comprises an optical light concentrator which is substantially transparent to light in the ultraviolet and/or visible region. The light concentrator has a light input region and a light output region along its length. Several light emitting diodes are positioned, one at each light input ends of a series of light guides. The light output end of each of the light guides abut the light concentrator along its length at the light input region, thus allowing concentrated light to be emitted along the length of the light output region of the concentrator such that the concentrated light is directed toward a photosensitive composition disposed on a substrate.